1. Field of the Invention
This invention relates to chemically modified polyaminosaccharides, processes of making such molecules, and their uses, particularly in wound healing. Specific aspects of this invention relate to chemically modified chitosans by a hydrocarbyl sultone compound.
2. Description of the Related Art
Polyaminosaccharides are of considerable interest in a number of fields ranging from medicine and agriculture to water treatment and cleaning products. However, the use of such molecules is somewhat limited by difficulties in dissolving them in water. Thus, various approaches have been used to increase solubility of polyaminosaccharides in water.
Previous processes often involved adding an organic or inorganic acid to polyaminosaccharides to render them soluble in water. However, the resultant solution has a very acidic pH, thereby decreasing its usefulness in many biological applications. Increasing the pH of the solution tend to cause precipitation of the polyaminosaccharides.
Other processes have involved addition of a sulfonic acid group to the molecules of polyaminosaccharides. However, these processes have oftentimes resulted in serious degradation of the starting materials. In addition, such processes tend to produce a pool of modified polyaminosaccharides with high variability in important properties, such as polymer length and degree of substitution. While such modified polyaminosaccharides may exhibit increased water solubility, their use in applications benefiting from precise knowledge of polyaminosaccharide properties is either severely curtailed or requires additional processing.
Chitosan, which may be represented by the following chemical structure, is a polyaminosaccharide of particular interest in a number of applications. Like many polyaminosaccharides, chitosan may be readily harvested from naturally occurring materials. The primary source of chitosan is discarded shells of lobsters and crayfishes or shrimps, although it may also be obtained from the shells of crabs and other crustaceans as well as from insect shells and fungi. Chitosan is normally non-toxic and is compatible with the tissues and skins of a variety of living organisms, including human beings. However, like many other polyaminosaccharides, chitosan exhibits only limited solubility in water.

Conventionally, the water solubility of chitosan may be increased by the addition of an acid. In addition, chitosan has also been sulfonated, which results in a molecule with a chemical structure similar to that of heparin, a powerful anticoagulant.
One previous process for sulfonating chitosan involves the use of chlorosulfonic acid as a sulfonating agent in an organic solvent of low polarity, such as pyrimidine. An organic base is subsequently added as an acid receptor for the sulfonation reaction. However, chlorosulfonic acid is not easy to use, and it sulfonates not only at the amino groups of the polyaminosaccharide, but also at other sites, such as the hydroxy groups. Besides, the need to supply an acid receptor complicates the reaction. Overall, the reaction is difficult to carry out and oftentimes results in a low yield of poorly characterized and unpredictable product.
Other known processes for sulfonating chitosan utilize alkyl sultones, such as 1,3-propane sultone, which are considerably easier to use than chlorosulfonic acid. In this aspect, reference may be made to, e.g., Kazuo Kondo et al., Journal of Chemical Engineering of Japan (1997), Vol. 30, No. 5, pp. 846-851, and a 2001 master thesis, entitled “Study on the semi-IPN of sulfonated polyurethane and chitosan,” which was authored by Yung-Hsin Lin, Chemical Engineering Institute, National Taiwan University. However, these processes involve the addition of alkyl sultones to chitosan in an aqueous acetic acid solution. The water molecules react readily with alkyl sultones in the aqueous solution and hydrolyzes them, thus resulting in a substantial loss of the same. Furthermore, the hydrolysis of the alkyl sultones results in the formation of a highly acidic alkylsulfonic acid, which then causes the degradation of the chitosan. As a result, these processes require the use of large amounts of alkyl sultone and chitosan but give very poor yields. Therefore, these processes are not economical for industrial applications. In addition, because of degradation, the length of the resultant alkylsulfonated chitosan varies widely, even when a uniform starting material is used. The degree of sulfonation of the amino group by the used alkyl sultone also varies widely and cannot be reliably predicted from the outset. Besides, when a chitosan was sulfonated based on the aforesaid processes, a predominant portion of bonding between the amino groups thereof and the alkyl sultone is actually ionic bonding, which can be readily disrupted by changes in the chemical environment. Although covalent bonding, which is much more stable, might occur, it is infrequent and does not represent a significant portion of the bonding between the amino groups of chitosan and the alkyl sultone. Finally, the applicants are unaware of any report in regard to the would-be properties and effects of the alkylsulfonated chitosan produced by these processes.
DE 3432227 A1 disclosed sulfopropyl derivatives of alkali chitins and chitosans of formula (I):H—[C6H11-o-pNO4(R1)o(R2)p]n—OH  (I)
wherein                R1 represents —CH2—CH2—CH2—SO3M (M=H, Na or K);        R2 represents —(C═O)—CH3;        n is integer from 50 to 10000;        o is a numeral from 0.05 to 4.0; and        p is a numeral from 0.01 to 1.0.        
DE 3432227 A1 disclosed that the aforesaid derivatives could be obtained from sulfonating alkali chitins or chitosans by 1,3-propane sultone with stirring (votexing) at a temperature of 10° C. to 80° C. and in the presence of an organic solvent (such as isopropanol (IPA) or acetone) for a period of 6 to 60 hrs. However, the three synthesis examples provided therein were conducted at a temperature of 25° C. or 45° C. for a period of 24 hrs or 48 hrs. respectively. In addition, in the three synthesis examples of DE 3432227 A1, IPA was the only solvent used in the sulfonation reaction, and acetone in fact was used as a precipitating agent in the product purification procedure.
According to the disclosure of DE 3432227 A1, one can obtain a composition of heterogeneous alkali chitin/chitosan molecules having a diversity of substitution patterns (i.e., the 1,3-propane sultone might be attached to either one or both of the —NH2 group and —CH2OH group of each saccharide monomer of alkali chitin/chitosan molecules, and acetyl groups might still remain in some saccharide monomers).
However, at least for safety reasons, a composition of heterogeneous molecules is not desirable for the manufacture of medicinal products, cosmetic products and the like. In addition, the biological properties of the products obtained in the three synthesis examples of DE 3432227 A1, if any, were not actually tested.
DE 3432227 A1 is totally silent to the use of organic solvents other than isopropanol, as well as the use of sulfonating agents other than 1,3-propane sultone. A possible reason for DE 3432227 A1 to select isopropanol as the only solvent for the sulfonation reaction may be that 1,3-propane sultone is highly reactive and will immediately be hydrolyzed to alkylsulfonic acid upon contacting with water. Since the commercially available isopropanol has a very low water content (<1%), hydrolysis of 1,3-propane sultone by water can be avoided. This assumption may also explain at least in part the rationale for DE 3432227 A1 to select so low a reaction temperature (25° C. and 45° C.).
However, the applicants found from experimentation that when chitosan is chemically modified by 1,3-propane sultone in the presence of isopropanol at 25° C. for 24 hrs, the recovered product has no significant increase in weight, indicating no or very low attachment of 1,3-propane sultone to the saccharide monomers of chitosan molecules. When the sulfonation reaction was conducted at 45° C. for 48 hrs, while the recovered product has an increase in weight, it will form a gel-like product when treated with an ammonia solution(aq), just like the alkylsulfonated chitosan obtained in the comparative example described below, in which a 2% acetic acid solution is used as the solvent (see infra). Therefore, it is very likely that the sulfopropyl derivatives of chitosans produced according to DE 3432227 A1 have molecular structures in which the sulfopropyl groups derived from 1,3-propane sultone are attached to the free amino groups primarily via ionic bonding instead of covalent bonding.
Accordingly, there is still a need in the art to develop new methods for sulfonating polyaminosaccharides, in particular chitosans, and to explore the potential of the thus-obtained products in a variety of applications, e.g., in the manufacture of a product selected from the group consisting of a personal care product, a food product, a cleaning product, an agricultural product, a cosmetic product, a medicinal product, a medical device, a fabric product, a product for water-treatment, and a biochemical product.